Reminder emails are put into place to let you know a couple days in advance that you have your due date approaching for your task(s).
Reminders are automatically generated once the Writer or Approver have been set in your Proposal's Workflow. If you have a task due on a specific date (say Tuesday), there will be an auto-generated reminder around midnight of the prior day (i.e. Monday). These are batched up and initially sent at the top of every hour, so you will get a single email with a summary of the due tasks.
An example of the email looks like this:
Our service (computer-speak for the program that looks for tasks) wakes up at a specific time each day, finds the tasks due in the next 24 hours, and generates the email reminder.